It is known that the treatment of nitrogen in waste-water is carried out in two steps:                a nitrification step during which the oxidation of the ammoniacal nitrogen, present in the effluent, to nitrite and then to nitrate is carried out by means of a biochemical reaction due to the action of autotrophic bacteria, and        a denitrification step during which the nitrate nitrogen is reduced to a lower oxidation state by means of a biochemical reaction using heterotrophic bacteria.        
Each of these two steps requires a certain number of conditions to be observed:                the nitrification step requires: a high sludge age, since the autotrophic biomass has a slow growth rate; a pH of between 6 and 8 with an optimal value of the order of 7, given that the growth rate of nitrifying bacteria decreases outside these pH values, and a dissolved oxygen content maintained at between 2 and 4 mg/l;        the denitrification step requires the following constraints to be observed: a low sludge age, given that the heterotrophic biomass exhibits rapid growth; a pH of between 6 and 8, with an optimal value of the order of 7; a very low dissolved oxygen content (anoxic conditions), given that the presence of oxygen inhibits denitrification, and a BOD5 sufficient to satisfy the organic carbon needs.        
It results from these constraints that the nitrification and denitrifications phenomena are entirely contradictory. It is the reason for which constructors of wastewater treatment plants have based their technique on spatial and/or temporal alternation of the aeration (nitrification) and anoxic (denitrification) phases.
It is known, moreover, that the denitrification rate depends on two essential parameters: firstly, the temperature, and secondly, the organic carbon available in the biological sludge, and therefore on the amounts of organic carbon provided by the effluent to be treated (at 15° C., the values are close to 2.5. to 3 g N—NO3/kg VSS/h).
At the current time, the denitrification process can be carried out in three different ways:                in an anoxia zone in free culture. The anoxic tank is located at the head of the treatment system and it is responsible for the denitrification. The supply of NO3− is provided by recirculation of the mixed liquor originating from the aeration tank, and the organic carbon needs are satisfied by the inlet of pretreated water. The denitrifying biomass is recirculated from the clarifier to the anoxic tank. The aeration tank ensures the nitrification and the additional elimination of the carbon-based pollution. The drawback of this configuration comes from the fact that it requires a recirculation of the order of 150 to 400% of the raw water flow in order to recycle the nitrates to be eliminated and so as to observe a sufficient C/N ratio. In general, the volume of the anoxic tank represents 25% of the total volume required for purification;        in the aeration-syncopating aeration tank, temporal alternation allows nitrification-denitrification in a single tank. In this case, the following optimum conditions should be observed: a sludge age of more than 10 days; a 30% increase in the aeration compared with the requirements of only elimination of the carbon-based pollution; a minimum anoxia time of the order of 8 to 10 h/day, and a sludge content of approximately 4 g VSS/l;        in a fixed-biomass installation (biofilter) which, in the same way as an anoxia zone, can make it possible to provide denitrification on the condition that air is injected in order to guarantee a homogeneous and controlled detachment of the excess biomass.        